Fuel cells, also referred to as electrochemical conversion cells, produce electrical energy by processing reactants, for example, through the oxidation and reduction of hydrogen and oxygen. Hydrogen is a very attractive fuel because it is clean and it can be used to produce electricity efficiently in a fuel cell. The automotive industry has expended significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Vehicles powered by hydrogen fuel cells would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
In a typical fuel cell system, hydrogen or a hydrogen-rich gas is supplied as a reactant through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied as a reactant through a separate flowpath to the cathode side of the fuel cell. Catalysts, typically in the form of a noble metal such as platinum (Pt) or palladium (Pd), are placed at the anode and cathode to facilitate the electrochemical conversion of the reactants into electrons and positively charged ions (for the hydrogen) and negatively charged ions (for the oxygen). In one well-known fuel cell form, the anode and cathode may be made from a layer of electrically-conductive gaseous diffusion media (GDM) material onto which the catalysts are deposited to form a catalyst coated diffusion media (CCDM). An electrolyte layer separates the anode from the cathode to allow the selective passage of ions to pass from the anode to the cathode while simultaneously prohibiting the reactant gases from crossing over to the other side of the fuel cell. The electrons generated by the catalytic reaction at the anode, which are also prohibited from flowing through the electrolyte layer, are instead forced to flow through an external electrically-conductive circuit (such as a load) to perform useful work before recombining with the charged ions at the cathode. The combination of the positively and negatively charged ions at the cathode results in the production of non-polluting water as a byproduct of the reaction. In another well-known fuel cell form, the anode and cathode may be formed directly on the electrolyte layer to form a layered structure known as a catalyst coated membrane (CCM). A membrane electrode assembly (MEA) may include, in one form, a CCM surrounded on opposing sides by respective anode and cathode GDMs, while in another form, a membrane made up of the electrolyte layer surrounded on opposing sides by respective anode and cathode CCDMs.
One type of fuel cell, called the proton exchange membrane (PEM) fuel cell, has shown particular promise for vehicular and related mobile applications. The electrolyte layer (also known as an ionomer layer) of a PEM fuel cell is in the form of a solid proton-transmissive electrolyte membrane (such as a perfluorosulfonic acid membrane, a commercial example of which is Nafion®). Regardless of whether either of the above CCM-based approach or CCDM-based approach is employed, the presence of an anode separated from a cathode by an electrolyte layer forms a single PEM fuel cell; many such single cells can be combined to form a fuel cell stack, increasing the power output thereof. Multiple stacks can be coupled together to further increase power output.
Durability is one of the factors that determine the commercialization of a fuel cell. For example, the desired life for a vehicle fuel cell is more than 5,000 hours. The high durability requirement is a challenge to the materials and structure development of the PEM for fuel cell applications. One or both of mechanical or chemical sources often contribute to the failure of the PEM in a fuel cell. Several strategies have been used to alleviate PEM mechanical degradation, including incorporating a reinforced layer (for example, an ionomer filled expanded polytetrafluoroethylene (ePTFE) matrix), introducing nanofiber (NF) materials into a PEM, and applying reinforced layers to reinforce the PEM externally. Known strategies to improve the chemical durability of a PEM include introducing precious metals (for example, the same Pt or Pd that can be used because of their strong catalytic activity for chemical reactions) into the PEM, and incorporating radical scavenger chemicals (for example, cerium (Ce) or manganese (Mn) through ion exchange or additives of salt and oxide format).